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Something Changed

 
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Old 05-26-2008, 12:39 PM
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Something Changed

I purchased a Sako 75 SS in 7mm recently. I purchased 150 pieces of new win brass, all the same lot#, weighed them and sorted out all +/- 1.5%, straightened out the mouth's, did the flashholes and primer pockets, and loaded up test loads with this brass for RL 19, RL 22, IMR 4831, and IMR 4350 at 3 loads each 1.0 gr apart up to the book max. I also purchased a brand new Zeiss Conquest 6.5x-20x 50mm and mounted it myself using Burris Rings that I received with the gun.

Of my test powders, using 160 gr Accubonds, I had 4 loads all under 1.0" so I loaded up enough for 3 more test groups for each of these. These included RL 22@ 61.0 gr, IMR 4831@62.0 gr, and 59.5 and 60.5 gr's of RL19. In the end I chose the RL19 as in 8 test groups with both poweder charges I had groups from .35 to .70 ave .52.

So I next loaded up more of the 60.5 gr's to further test, with no variations to the load or gun except that I had run out fo the Nosler's and had to purchase a new lot. The cold barrel foul shots went into just under an inch, then the first group was again at .5". The next groups started with two less than .5", but the third was over 1.5" away. I thought it might be a flyer, so I shot a fourth, again 1.5" in a diff direction. Two more groups provided 1.5 and 2.25" groups.

After some advice here I checked the action screws, which actually turned 1/3-1/2 turn fairly easily, then I checked the scopre mounts and ring screws, so I tightened them slightly. This provided no benefit. Then I switched scopes to a spare Bushell Elite 3200 3x9, to no benefit. I bought a new box of Accubonds, just in case it was a bad batch, but no help. I had one group at .75", but all others were between 1.5" and 2.5".

My groups typically have two of three that are good and a third that doesn't seem to have a typical pattern and is usually .75-2.0" away. It isn't necessarily vertical or horizontal. Before they started to open up, two of three on all 8 first groups mentioned had two in the same hole with the third .3-.7 inches away.

I'm new to this and was very excited to get such consistent loads, and now I'm quite depressed. It seems like something went wrong all of a sudden. I dont' think it's me, as I can take my kids gun that shoots consistently under an inch and consistently shoot under an inch with it. I can understand the occasional flyer, but now one out of three.

Should I take the gun to a smith for a bedding job. The barrel seems floated with no resistance on a dollar bill and I can't easily push the stock to meet the barrel.

Additionally I thoroghly cleaned the barrel before each use, fouled with 3-4 shots, allowed about two minutes after the first shot, about 3-4 after the second and at least 5 minutes in the shade with the action open between groups.

Please help with some advice?
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Old 05-26-2008, 05:45 PM
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  #3  
Old 05-26-2008, 08:57 PM
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I did purchase one. These loads seem fairly consistent. I believe the slowest speed was 2791 and the fastest was 2848, with most in the 2820 to 2830 range. I did think these were quite slow compared to the book which rated this load at around 3010. By comparison the 4831 loads seemed to have spreads of up to 110 fps, with the max being over book value and the ave around book estimate. The fairly consistent readings are another sign to me that something went wrong all at once.
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Old 05-26-2008, 09:36 PM
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Quote:
Originally Posted by bkondeff View Post
I did purchase one. These loads seem fairly consistent. I believe the slowest speed was 2791 and the fastest was 2848, with most in the 2820 to 2830 range. I did think these were quite slow compared to the book which rated this load at around 3010. By comparison the 4831 loads seemed to have spreads of up to 110 fps, with the max being over book value and the ave around book estimate. The fairly consistent readings are another sign to me that something went wrong all at once.

Ok. Now we need more information. You mean on any given load, you have 110 fps extreme spread? If so, that would indicate you might have a standard deviation of 50 fps or more. A load with that kind of inconsistency might shoot good one day but it will not usually repeat it's performance again. Also, you need to look for nodes or loading windows (also called sweet spots) in the overall load ladder. These are indicated by low deviations were the ignition of the powder column is at a most efficient point. There are usually two nodes. ONe at a low pressure, and one at a high pressure. In my experience of working on many 7mm mags is that they usually have the best performance at the higher pressure node which is around 3000 to 3050 fps with a 160 grain bullet. So my guess is that you are inbetween the two nodes right now and therefore have crappy, inconsistent performance.

Then once you find a powder charge that yeilds low variations in speed, then tinker with seating depths. Accubonds are notorious for liking quite a bit of "jump" to the riflings. So load them to fit in the magazine (if being used for hunting) and then seat them deeper into the case in .005" increments. In this method, you will get good velocity uniformity and shrink the groups up a bit.

I just also saw that your gun is not bedded. You really ought to get this done for accuracy consistency too.
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